How can read file with structure contains fixed string element?

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C#

string

file

Hi all.
I am trying to read a binary file with a structure that contain fixed string element. But it doesn't work fine.
I know dat file with structure as below. The DAT file was created by other application that could code by Perl or VB 6 or any language.

FILE.DAT

ID Int (2 Byte)
Name String (36) 
Double Count
Flag String(1)

Here is my code.

public struct sFile
        {
            public short ID;
            [MarshalAs(UnmanagedType.ByValTStr, SizeConst=36)]
            public string Name;
            [MarshalAs(UnmanagedType.R8)]
            public double Count;
            [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 1)]
            public string Flag;
        }

Load dat file to Datatable Method.

public DataTable sFileLoad(string pztablename, string pzfilename)
       {
           sFile iFile = new sFile ();
           DataTable dtt = new DataTable();
           if (sfRead == null) OpenSFile(pzfilename);
           if (sfRead.EOF) OpensFile(pzfilename); // Open dat file method.

           dtt.Columns.AddRange(new DataColumn[] {
               new DataColumn("ID", typeof(Int16)),
               new DataColumn("Name", typeof(String)),
               new DataColumn("Count", typeof(decimal)),
               new DataColumn("Flag", typeof(String)))
               });
           try
           {
               while (!sfRead.EOF)
               {
                   sFile = (sFile)sfRead.GetNextStructureValue();

                   dtt.Rows.Add(iFile.ID,
                                iFile.Name,
                                iFile.Count,
                                iFile.Flag);

               }


           }
           catch (Exception Ex)
           {
               MessageBox.Show(Ex.Message.ToString());
           }

           return dtt;

       }

There is a problem while reading file is the flag field have "" value.
if I did it with vb6 it works fine.
Value of Flag field is "S", "M", "T"...

Help me fix this code.
Many thanks